This invention is generally related to the production of 1,3-dihydro-1,3-dioxo-5-isobenzofuran carboxylic acid more commonly known as trimellitic anhydride (TMA). More specifically, the invention is related to the addition of a mineral acid or salt thereof in the production process of trimellitic anhydride.
Trimellitic anhydride (TMA) is a commercial chemical intermediate useful in various areas including plasticizers and specialty coatings.
TMA is commonly produced by the oxidation of pseudocumene (1,2,4-trimethylbenzene) to trimellitic acid (1,2,4-benezenetricarboxylic acid) followed by dehydration to form the anhydride. The oxidation of pseudocumene to trimellitic acid is preformed using a metal catalyst system.
After oxidation of pseudocumene and dehydration to form the anhydride, the resulting crude TMA is fed into a fractionation column for purification. In this distillation step, the crude TMA is separated into a relatively low boiling point fraction and a less volatile bottoms fraction. The low boiling point fraction is recovered from the top of the column as purified TMA while the bottoms fraction falls to the bottom of the column where it is recycled back to the column through a reboiler. The bottoms fraction contains TMA and various impurities including: 1) multivalent metals from the catalyst system used for oxidation of pseudocumene and 2) aromatic polycarboxylic acids byproducts. Since these impurities are concentrated in the bottoms fraction as it is recycled, some amount of the bottoms portion is periodically or continuously removed though a bottoms purge stream and combined with other waste streams to be incinerated.
A problem encountered during the distillation step is that the viscosity of the bottoms fraction increases as the bottoms are continuously recycled and the more volatile TMA is removed. If unchecked, the viscosity reaches a point where fouling or plugging of the fractionation column occurs. More commonly a high viscosity in the bottoms leads to fouling or plugging of the bottoms purge stream and/or the reboiler. Such fouling or plugging means a significant economic loss due to: 1) the time and expense required to clean and return the column to an operable state and 2) the loss of production capacity. One solution to the problem has been to maintain high levels of TMA in the bottoms fraction in order to reduce viscosity. Unfortunately, maintaining high levels of TMA in the bottoms fraction results in a yield loss, i.e. lower recovery of TMA from the top of the column since high levels of TMA exit the fractionation column though the bottoms purge stream instead. Accordingly, it would be desirable to discover a way of reducing the viscosity of the recycled bottoms fraction contained in a fractionation column for TMA, thereby reducing the amount of TMA required in the bottoms to prevent fouling/plugging and ultimately increasing the amount of TMA recovered from the top of the column.
The present invention solves the problem of increasing the level of TMA recovery in a fractionation column while also preventing fouling/plugging of the column resulting from the high viscosity of the bottoms fraction. The present invention solves this problem by the addition of a mineral acid or salt thereof to the process. It has been surprisingly discovered that the addition of a mineral acid or salt thereof reduces the viscosity of the bottoms and permits the use of lower levels of TMA in the bottoms fraction necessary to prevent fouling/plugging.
The present invention is a method for making trimellitic anhydride comprising:
a) oxidizing pseudocumene in the presence of a solvent and a catalyst comprising multivalent metals to produce trimellitic acid;
b) crystallizing and filtering said trimellitic acid to produce a crude trimellitic acid cake having impurities comprising said multivalent metals;
c) dehydrating said crude trimellitic cake to form crude trimellitic anhydride; and
d) distilling said crude trimellitic anhydride in a fractionation column;
wherein a viscosity modifier is added after step a) but before step d) so that said viscosity modifier is present in said bottoms fraction.
While it is not intended that this invention be bound or limited by any particular theory, it is believed that the viscosity of the bottoms fraction is related to the presence and interaction of impurities in the crude TMA. Impurities in the bottoms fraction comprise multivalent metals that are present from the metal catalyst system and organic by-products from the oxidation of pseudocumene to trimellitic acid. Multivalent metals that are present as impurities may include cobalt(II), cobalt(III), cerium(III), cerium(IV), manganese(II), manganese(III), zirconium(IV). Organic impurities may also include various aromatic polycarboxylic acids and anhydrides thereof. Examples of these aromatic acids are trimellitic acid, 1,2,3,4-tetracarboxylic acid, 1,2,4,5-tetracarboxylic acid and 1,2,3,5-tetracarboxylic acid.
It is theorized that the multivalent metals and the acids and anhydrides thereof form polymeric complexes which increase the viscosity of the bottoms fraction. It is believed the formation of these polymeric complexes results from the ability of the multivalent metals to form complex salts with the aromatic acids and an hydrides thereof. Since aromatic polycarboxylic acids and anhydrides thereof provide multiple sites for the formation of salts, it is believed that polymer structures having repeating unit represented by -(A-B)n- are formed, wherein A-B is a salt formed by a multivalent metal(A) and a acid(B). It is further hypothesized that high molecular weight and highly branched polymers are formed which result in a higher viscosity of the bottoms fraction.
It is believed that a mineral acid or salt thereof interferes with the formation of such polymers by forming lower molecular weight and/or less branched salts of the mineral acid with the multivalent metals. Accordingly, it is thought that the addition of a mineral acid reduces viscosity of the bottoms fraction by hindering or preventing the multivalent metals and aromatic acids from forming polymeric complexes.